[new feature] add Collapse component (#674)

test/components/collapse.vue

@@ -0,0 +1,21 @@
+<template>
+  <van-collapse v-model="active" :accordion="accordion">
+    <van-collapse-item title="a" name="first">content</van-collapse-item>
+    <van-collapse-item title="b">content</van-collapse-item>
+    <van-collapse-item title="c">content</van-collapse-item>
+  </van-collapse>
+</template>
+
+<script>
+export default {
+  props: {
+    accordion: Boolean
+  },
+
+  data() {
+    return {
+      active: this.accordion ? '' : []
+    };
+  }
+};
+</script>